Antioxidants Explained In Simple Terms

Understanding antioxidants

You have doubtless heard of antioxidants and that things like green tea, berries, kale and dark chocolate are full of them. Many nutritionists and health professionals recommend everyone to be eating more of these “superfoods” but what exactly are antioxidants? How do you know if you getting enough of them and what is the best way to include more of them into your diet?

Why do we need antioxidants?

Oxidation is when a substance reacts and combines with oxygen. Electrons are transferred from one molecule to another so that when one loses electrons (‘oxidation’), another gains electrons (‘reduction’). This chemical reaction is called an oxidation-reduction, or ‘redox’ reaction, shown below (Left to Right: oxygen, superoxide, hydrogen peroxide, hydroxyl radical)

Redox reactions change the chemical properties of the molecules involved and this can alter the structure of cell membranes and affect the action of essential proteins. An excess of reactive oxygen species (e.g., hydrogen peroxide, hydroxyl radical) can damage cells, induce inflammation and contribute to the progression of many diseases.

What are free radicals?

• Free radicals are reactive oxygen species that have an odd number of electrons; (atoms usually like to have all their electrons in ‘pairs’). They are a natural by-product of many cellular processes, including energy metabolism. Having an unpaired electron makes free radicals unstable. They will react with other molecules around them and try to grab an electron from anything they can to balance themselves out. When unchecked, such reactivity can cause damage to cells and DNA
• Free radicals can be both helpful and harmful to the body. For example, free radicals are harnessed by white blood cells to kill invading microbes and are an important feature of normal immune defences. Exercise increases the production of free radicals, which help to break down and dispose of damaged muscle cells to allow for the growth of new ones
• Free radicals can enter the body through exposure to pollution, chemicals, alcohol, radiation, sun exposure, and the foods we eat, particularly cooking oils and fried foods
• Free radical accumulation, either due to excess production or environmental exposure, is termed ‘oxidative stress’. Oxidative stress is implicated in a number of disease processes involving inflammation, including rheumatoid arthritis, heart disease, cancer, diabetes and even ageing

What types of antioxidants are there?

Phytochemicals

Phytochemicals (or phytonutrients) are responsible for the pigments in plants which give them their bright yellow, orange, red and purple colours. Other phytochemicals contribute to the bitter and aromatic flavours of some plants. They are also bio-active compounds that help to protect plants from damage caused by bacteria and fungi. They are not essential for human growth and development but phytochemicals do play a role in the maintenance of good health, including the prevention of chronic diseases.

Not all plant phytochemicals have antioxidant properties, some have adverse as well as beneficial effects (e.g., poisons). However, a great deal many are potent antioxidants which are broadly divided into groups that include terpenoids, polyphenols, and sulphur-containing compounds. 

Some examples are:

Polyphenols

• Quercetin (onions)
• Flavanones (capsicum pepper) 
• Anthocyanidins (red/purple fruits and vegetables)
• Catechin (green tea)
• Tannins (red wine, cocoa)
• Curcumin (turmeric)

Terpenoids

• Carotenoids (carrots, sweet potato, pumpkin)
• Lycopene (tomatoes)
• Phytosterols (seeds, seed oils, legumes, especially soy)

Thiols (sulphur-containing compounds)

• Glucosinolates (cruciferous vegetables such as broccoli, asparagus, Brussels sprouts, cauliflower, horseradish, radish and mustard)

Antioxidant nutrients

Vitamin C

Vitamin C, also known as ascorbic acid, is an extremely important water-soluble vitamin that plays a role in collagen production, immunity, and formation and maintenance of healthy bones and other tissues. It is abundant in a wide variety of fruits and vegetables, particularly citrus fruits.

As well as being an essential micronutrient, vitamin C is also a powerful free radical scavenger (antioxidant). An example is the use of lemon juice (high in antioxidants) in fruit salads, to prevent fruits such as apples and bananas (low in antioxidants) from turning brown. Vitamin C (labelled as ascorbic acid) is also widely used as a preservative in a range of commercial food products.

Vitamin C (and other antioxidant vitamins) protects the heart and cardiovascular system by stopping a process called lipid peroxidation - a free-radical chain reaction involving fatty acids that can lead to hardening of the arteries (atherosclerosis).¹

Low intakes of vitamin C have been linked to:

• High blood pressure (hypertension) 
• Increased inflammation and oxidative stress
• An almost 40% increased risk of heart developing disease  

Vitamin E

Tocopherol refers to several compounds collectively known as vitamin E. Alpha-tocopherol is the only form of vitamin E that is stored in human tissues. It helps to maintain the structural integrity of our cells by neutralising free radicals that can interact with the cellular membrane, proteins and DNA. 

The human brain is highly susceptible to oxidative stress, which increases as we age. Vitamin E offers some protection against neurodegeneration in the elderly as high blood concentrations of alpha-tocopherol have been linked to better cognitive performance and healthier brain ageing. ²

The primary source of vitamin E in the diet is seeds and seed oils used for cooking including:

• Sunflower seeds and oil
• Soybeans 
• Safflower oil
• Rapeseed oil

These oils are also a major source of n-6 polyunsaturated fatty acids (PUFAs). Vitamin E is naturally present together with PUFAs in foods and helps to regulate oxidation, however, if seed oils are used for cooking at high temperatures this can reduce the effects of antioxidants in vitamin E. Therefore, seed oils (sometimes misleadingly referred to as ‘vegetable’ oils) when used for frying are not rich in vitamin E. 

Beta carotene (provitamin A)

Some carotenoids (another family of phytochemicals) are also classed as ‘pro-vitamins’ because they can be converted into vitamin A (retinol) in the small intestine. Beta-carotene (associated with the orange pigment in carrots) is perhaps the most well-known pro-vitamin and is a major source of retinol in the diet.

The antioxidant properties of beta-carotene are thought to protect against skin damage from ultraviolet radiation which can lead to cancers.³

Overall, vitamins C, E and beta-carotene act directly as antioxidants. However, other essential micronutrients such as copper, manganese and selenium, as well as certain amino acids, can be classed as antioxidants for their hand in helping the body produce other enzymatic antioxidants. There are also potentially many hundreds or even thousands of different substances that can be classed as antioxidants.

Food sources of antioxidants

Plant-based foods supply an average of 64 times more antioxidants than animal foods. Some exotic berries and traditional plant medicines have antioxidant values many magnitudes higher than more common foodstuffs but the best way to boost your intake is by eating a wide variety of fruits, vegetables, herbs and spices each day. 

Herbs and spices have traditionally been used in cuisines for flavouring, but their excellent antioxidant properties may also help prevent illness.

Teas are a good choice too - dry green tea is 30-40% polyphenols and many of the compounds in both black and green teas have been shown to inhibit the growth of tumour cells.⁴ 

Cautions before use

The benefits of supplemental antioxidants however have not been confirmed, and some high-dose antioxidant supplements have even been found to have a negative effect on human health.⁵ There have also been concerns over the use of synthetic antioxidants as food preservatives. Some studies in rodents suggest that some antioxidants (e.g., butylated hydroxytoluene (BHT, E321) and butylated hydroxyanisole (BHA, E320)) may be carcinogenic, although detrimental effects in humans have not been found.⁷  Still, both compounds are currently under the scrutiny of various researchers and food regulation authorities.

Summary

Lifestyle factors such as smoking, alcohol, and processed food intake expose us to free radicals which cause oxidative stress. Prolonged oxidative stress is understood to play a role in the progression of many chronic diseases and has been proposed as a mechanism of ageing. 

Consuming plenty of antioxidant-rich foods can help protect against chronic diseases. Higher intakes of plant antioxidants are associated with a lower risk of cancers, heart disease, and stroke. The best way to increase your intake is to eat a variety of antioxidant-rich plant foods. A comprehensive list of more than 3,100 foods, beverages, spices, herbs and supplements and their total antioxidant content can be found here.